The present invention concerns a method of measuring the cutaneous electrical resistance of a patient and, more particularly, a method of this kind used on a patient undergoing transdermal administration of medication assisted by an ionophoresis current.
Many devices have been designed for ionophoretic administration of medication. That described in French patent application no 96 04735 filed Apr. 16, 1996 by the Applicant may be cited as one example. Administration in this way implies the use of an active principle taking an ionic form susceptible, under the action of an electric field, to pass through the skin of the patient. The field is established between two adjacent electrodes applied to he skin of the patient. The administration of the medication develops conventionally in accordance with a time programme including periods of administration optionally separated by periods of non-administration during which the ionophoresis, or xe2x80x9ctherapeuticxe2x80x9d, current is cut off. During periods of administration the current may be set at predetermined values, for example 500 xcexcA, 750 xcexcA, that can differ from one period to another. The total duration of an administration programme is routinely measured in hours, even in days.
The application of an electric current to a surface element of the skin of a patient for periods of time this long can modify the characteristics of the skin, and even cause lesions, from slight superficial oedema to deep burns, accidents that obviously have to be prevented by constant monitoring of the status of the skin surface element subjected to the ionophoresis current.
This monitoring is routinely effected by monitoring the electrical resistance of this surface element of the skin of the patient, as a modification of the structure of the skin, due to the imminent appearance of a lesion, is manifested in a variation in said cutaneous electrical resistance.
Conventionally, to measure this electrical resistance the electrical voltage between the electrodes applied to the skin of the patient is noted at the time of the measurement. These electrodes are part of an ionophoresis assisted transdermal administration device that includes means for adjusting the ionophoresis current.
Knowing the voltage between the electrodes and the current, a measured value for the cutaneous resistance of the patient within the current tube limited by the two surface elements of his skin covered by said electrodes can be deduced, simply by application of Ohm""s law. In the following, said electrical resistance is called xe2x80x9ccutaneousxe2x80x9d resistance of the patient.
Incidentally, note that this voltage also enables diagnosis of malfunctions of the device, for example short-circuiting of the skin by sweat on the surface of the skin, insufficient hydration of a reservoir of active principle joined to an electrode, etc, etc, . . .
The voltage measurement referred to above does not enable reliable conclusions to be drawn from the observed variations in the measured values, however, because the current/voltage characteristic of the skin is not linear, as can be seen from the graph of this characteristic represented in FIG. 1 of the accompanying drawing.
This graph, established with electrodes having a surface of 4 cm2 and from measurements taken without any significant modification of the structure of the skin appearing, shows that, depending on the value of the ionophoresis current I at the times the voltage is measured, the cutaneous resistance as measured using said law can take different values even if the structure of the skin has not deteriorated at all from one measurement to the other. It therefore appears that the voltage measurements mentioned hereinabove cannot provide a reliable diagnosis of the imminent appearance of a lesion in the skin surface element subjected to the ionophoresis current.
An aim of the present invention is to provide a method of measuring the cutaneous electrical resistance of a patient undergoing ionophoresis assisted transdermal administration of medication that can reliably diagnose the imminent appearance of a lesion of this kind.
Another aim of the present invention is to provide a method of the above kind that can also diagnose the occurrence or the existence of malfunctions of the device used to perform the ionophoresis assisted transdermal administration of the medication.
These aims of the invention, and others that will emerge from a reading of the following description, are achieved with a method of the type described in the preamble to this description, this method being noteworthy in that the ionophoresis current is temporarily adjusted to a predetermined value corresponding to a point on a rectilinear portion adjoining the origin of the current/voltage characteristics of the skin of the patient, the voltage then present between the two electrodes supplying the ionophoresis current to the skin of the patient is measured and the measured cutaneous resistance is obtained from the measured voltage and the predetermined value of the ionophoresis current.
As will be shown in more detail hereinafter, by setting the value of the current in this way at the time the voltage is measured, the problem of the non-linearity of the current/voltage characteristic of the skin likely to interfere with the diagnosis of the imminent appearance of a lesion on the skin of the patient is avoided.
According to another feature of the method of the invention, said predetermined value of the ionophoresis current is a value corresponding to a substantially zero flux of the medication through the skin of the patient.